Antibiotics are overprescribed in pediatric populations, and antibiotic-exposed children can have signs of gut microbiota imbalance (dysbiosis) for months to years afterwards. Gut microbiota play key roles in immune development and function. Correspondingly, dysbiosis and early childhood antibiotic exposure have been implicated as potential causes of juvenile idiopathic arthritis (JIA). Studies on antibiotics and JIA have been restricted to two European populations of mostly young children, and their findings need replication and deeper examination in broader, more diverse populations. The use of conventional and biologic disease-modifying antirheumatic drugs (DMARDs) has vastly improved outcomes for children with JIA, but response to specific drugs is variable and difficult to predict. Methotrexate (MTX), the most common DMARD used to treat JIA, is toxic to certain commensal bacteria found in higher abundance in children with JIA. Moreover, early evidence suggests that certain gut microbiota can metabolize MTX, and adults with rheumatoid arthritis who respond poorly to MTX may have different gut microbiota from responders. There is a critical need to understand better how a potentially modifiable factor?antibiotic exposure?affects variability in JIA incidence, phenotype, and therapeutic response to DMARDs such as MTX so that children receive appropriate, effective medicines. This project's long-term goal is to ensure that all children with JIA receive effective and safe treatment and to identify new modalities for JIA treatment and prevention. The overall objective of this proposal is to understand how antibiotics affect the risk of developing JIA and the response to standard JIA treatments. This project will (1) test how patterns of antibiotic exposure relate to incident JIA and JIA phenotype and (2) determine whether recent antibiotic exposure in children with JIA starting DMARDs is associated with early changes in therapy. The central hypothesis is that antibiotic exposure increases the risk of JIA and impairs the therapeutic response to MTX more than to other DMARDs, such as tumor necrosis factor inhibitors. The project team will use administrative claims data to conduct retrospective cohort studies on the effects of antibiotic exposure in large, diverse general pediatric populations (Aim 1) and in patients with JIA starting MTX and other DMARDs (Aim 2). The proposed research will yield novel and important information about the potential risks of antibiotics in relation to the most common pediatric rheumatic disease and standard antirheumatic drugs. This research will produce critical clues about underlying mechanism for potentially differential responses to specific DMARDs. Furthermore, this project will lay important foundations for future interventions to limit infections and inappropriate antibiotic use in children and potentially to manipulate microbiota in order to treat or prevent JIA and promote DMARD effectiveness and safety.